1. Field of the Invention
This invention relates to a cooling device of a magnetron, which is used in a microwave oven to radiate heat of a high temperature produced during oscillating operation of the magnetron for generating an electromagnetic wave.
2. Description of the Prior Art
Generally, a magnetron for generating a microwave is, as shown in FIG. 1 of the accompanying drawings, a kind of a diode comprising an anode cylinder (referred to as an anode) 1 having a plurality of radially extending vanes 1a mounted on its inner periphery, and a direct-heated filament (referred to as a cathode) 2 disposed axially in centered relation within a yoke 11 serving as a frame and surrounded by the anode cylinder. In addition, the magnetron comprises a magnetic circuit section including an annular permanent magnet 4 and a magnetic pole 5. The magnetic circuit section applies magnetic flux into an active space S defined between the anode and the cathode. The magnetron also includes an output section including an antenna lead 6, an antenna seal 7, an antenna ceramics 8 and an antenna cap 9. The output section emits microwave energy transferred to the anode cylinder 1 to a cavity of a microwave oven, i.e., the exterior of the magnetron, through a waveguide (not shown). The magnetron includes a radiation section including a plurality of radiation fins 3 disposed in parallel. The radiation fins are equally spaced-apart in relation to each other and are located between the outer periphery of the anode cylinder 1 and the inner peripherey of the yoke 11 to radiate heat of a high temperature generated during oscillating operation of the magnetron. The magnetron includes a filter circuit for preventing an unnecessary component of the microwave produced in the active space between the anode cyliner 1 and the filament 2 from back-flowing to a power source.
In the magnetron thus constructed, when the filament 2 is energized, thermions are emitted from the filament into the active space S. The thermions effect a cycloidal movement as they are subjected to the force of an electric field induced between the vanes 1a and the filament 2 and the magnetic flux applied into the active space S by the magnetic pole 5 of the magnetic circuit section. As a result, the accelerated thermions generate the microwave energy which will be received by the vanes 1a. The microwave energy transferred to the vanes is then emitted through the antenna lead 6 of the output section into the cavity of the microwave oven via the waveguide. At this time, heat of a high temperature is generated as the thermions with the energy applied by the electric field impinge against the vanes of the anode cylinder. In order to radiate the heat to the outside, therefore, it is necessarily required to dispose a plurality of radiation fins 3 on the outer periphery of the anode cylinder 1.
As shown in FIG. 1, each of the radiation fins 3a is secured at its one end to the outer periphery of the anode cylinder 1 and is brought at its opposite end into engagement with the inner surface of the yoke 11, thereby radiating the heat of a high temperature generated due to collision of the theremions with the vanes. In order to provide a path for radiating the heat, the conventional radiation fins are made of aluminum or alloys thereof and disposed in parallel, equally spaced relation to each other, as shown in FIG. 2.
Describing the prior art in more detail the outer surface of the anode cylinder 1 with a height H permits a plurality of the radiation fins 3 to be disposed thereon in parallel, vertically spaced-apart relation to each other. When the design height H is approximately 26 mm, each fin has a thickness t of approximately 0.6 mm, and a total of 4 to 7 fins are disposed at equal distances apart in the range of about 3.5 to 6 mm.
In the cooling device of the prior art comprising the equally spaced radiation fins, when the heat of a high temperature is generated due to collision of the thermions with the vanes, the radiation fins secured to the outer surface of the central portion of the anode cylinder (having the vanes mounted on its inner periphery) have a temperature of about 10.degree..about.30.degree. C.; higher than the temperature of the fins disposed on both ends of the sides of the anode cylinder. There is a difference in temperature between the different fins disposed axially on the anode cylinder. Therefore, when the magnetron is continuously operated for a long time, the fins disposed centrally on the outer peripherey of the anode cylinder tend to be deformed by the heat of a higher temperature transferred continually to them. As a result, the magnetron of the prior art may undergo a degrading of an operating characteristic, resulting in defective operation due to an abnormal temperature rise.